Vapour discharge lamps

ABSTRACT

The invention relates to improvements in low-pressure vapour discharge lamps, for example sodium vapour discharge lamps. According to the invention a discharge tube for such a lamp is provided with an exhaust tube which extends into the discharge tube. The exhaust tube is closed at its tip within the discharge tube and is adapted to constitute a reservoir for vapourisable substance, such as sodium. Communication between the reservoir and interior of the discharge tube is provided by an aperture in the wall of the exhaust tube. In lamps made with such discharge tubes, the reservoir helps to ensure even distribution of vapour within the discharge tube, thereby improving the intensity and uniformity of light emitted by the lamp.

United States Patent Scott 1 Jan. 8, 1974 .VAPOUR DISCHARGE LAMPS [57]ABSTRACT v [75] mentor: Kenneth Scot" Lane England The invention relatesto improvements in low-pressure [73] Assignee: Thorn ElectricalIndustries Limited, ap discharge lamps, for eXHmPIE Sodium p r London,England discharge lamps.

According to the invention a discharge tube for such a [22] Filed 1973lamp is provided with an exhaust tube which extends PP 322,745 into thedischarge tube. The exhaust tube is closed at its tip within thedischarge tube and is adapted to [52] Us. Cl 313/174 313/225 313/227constitute a reservoir for vapourisable substance, such 313/229 316/24as sodium. Communication between the reservoir and [51] Int. Cl H01j61/24 interior of the discharge tube is provided by an [58 Fieldo'r'siiifjfIIIIIIIIIIIII 31521"?! 225 227 aperture in the 110mm exhausttube- 313/229; 316/24 In lamps made with such discharge tubes, thereservoir helps to ensure even distribution of vapour within the [56]Refe ces Cit d discharge tube, thereby improving the intensity andUNITED STATES PATENTS uniformity of light emitted by the lamp.

3,548,240 12/1970 Spiessens 3l3/l74 Primary ExaminerHerman Karl SaalbachAssistant Examine r-Darwin R. Hostetter Attorney-Solon B. Kemon et al.

4 Claims; 1 Drawing Figure 1 VAPOUR DISCHARGE LAMPS The presentinvention relates to electric discharge lamps, for example low-pressuresodium-vapour dis charge lamps.

A low-pressure sodium vapour discharge lamp comprises a discharge tubecontaining electrodes and a fill of inert gas and sodium. The dischargetube comprises a light-transmitting envelope and is normally surroundedby an evacuated jacket. ln manufacture, the discharge tube is usuallyexhausted of air and filled with gas and sodium through an exhaust tubeformed in the wall of the envelope, the external tip of the exhaust tubethen being sealed. In operation of the lamp, sodium in the vapour phaseis ionised in the discharge and gives rise to almost all of the lightemitted by the lamp. The instensity and uniformity of the emitted lightdepend on the concentrationof sodium vapour and its distribution in thedischarge tube.

In order to ensure even distribution of sodium vapour, it has been thepractice to provide the walls of the discharge tube with suitablydispersed dimples. During operation of the lamp the dimples are slightlycooler than the walls of the discharge tube and act as reservoirs ofsolid or liquid sodium. A disadvantage of this is that many dimples (forexample, ten) are required, and their provision complicates themanufacture of the lamp. A similar problem arises with lamps employingother vapourisable substances for example cadmium, potassium, andlithium.

According to the present invention the discharge tube of a low-pressurevapour discharge lamp, comprises a light-transmitting envelope and anexhaust tube which extends into the envelope and terminates within theenvelope in a closed end adapted to constitute a reservoir forvapourisable substance, the reservoir being in communication with theinterior of the envelope through an opening in the side wall of theexhaust tube.

The invention will now be described by way of. exampie, with referenceto the accompanying drawing which represents a fragmentary longitudinalsection through a linear discharge lamp.

The lamp consists of a linear discharge tube comprising an envelopecontaining a cathode 11 near one end and an anode 12 near the other end,together with an exhaust tube 13. The discharge envelope 10 issurrounded by a jacket 14; the space between the jacket 14 and thedischarge envelope is exhausted. Each electrode is connected by twoelectrically conducting wires 15 to pins outside the jacket 14.

The end 18 of the exhaust tube 13 within the envelope 10 is closed andthe outerv end of the tube is sealed at the tip 22. The end 18 withinthe envelope l0 lies near one of the electrodes, which in the drawing isthe cathode 11. An aperture 17 in a wall of the exhaust tube 13 withinthe envelope 10 provides for communication between the end of the tubeand the interior of the envelope 10. The discharge tube contains a fillof sodium vapour and inert gas at low pressure, and sodium 19 not in thevapour phase is contained in the reservoir constituted by the closed end18 of the exhaust tube. I

During manufacture of such a lamp, the discharge tube is exhaustedthrough the exhaust tube 13, the external tip 22 of which is at thattime open. The gaseous filling is then introduced through the exhausttube 13 and passes through the aperture 17 into the envelope 10. Liquidsodium is also introduced into the exhaust tube 13 and is caused tocollect at the closed end 18 of the exhaust tube inside the envelope.After the filling operations the exhaust tube is sealed at its tip 22outside the envelope in the conventional way.

In the completed lamp, sodium not in the vapour phase is maintained inthe reservoir near the cathode. The amount of sodium vapour in the lampdepends upon the dimensions and position of the exhaust tube.

Lamps of like construction can be used with other vapourisablesubstances, such as lithium, potassium and cadmium.

What is claimed is:

1. In a low-pressure vapour discharge lamp comprising alight-transmitting envelope, at least two spaced electrodes in saidenvelope, and an exhaust tube communicating with the envelope, theimprovement comprising: said exhaust tube extending into said envelopeand having an end within the envelope that is closed and adapted toconstitute a reservoir for vapourisable substance, said reservoir beingin communication with the interior of the envelope through an opening inthe exhaust tube.

2. A low-pressure vapour discharge lamp according to claim 1 in whichthe reservoir is near one of the electrodes of the lamp.

3. A low-pressure vapour discharge lamp according to claim 1 in whichthe reservoir contains a vapourisable substance selected from the groupconsisting of lithium, sodium, potassium, cadmium and mixtures thereof.

4. A discharge tube for a low pressure vapour discharge lamp comprising:a light-transmitting envelope; an exhaust tube extending throughthe wallof said envelope and being sealed to said envelope wall; said exhausttube having an end within said envelope that is closed and adapted toconstitute a reservoir for vapourisable substance, and said exhaust-tubefurther having a wall within said envelope formed with an aperturepermitting communication between said reservoir and the interior of saidenvelope.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,784,863 Dated January 8, 1974 Inventor 5) Kenneth SCOTT It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

[30] Foreign Application Priority Data Jan. 28, 1972 Great Britain4193/72 Signed and sealed this 6th day of August 1974.

(SEAL) I Attest:

MCCOY M. GIBSON, JR. v c. MARSHALL DANN Attesting Officer 7 Commissionerof Patents FORM PO-IOSO (10-69) USCOMM-DC 603764 69 u.s GOVERNMENTPRINTING orHcE 1 9 o3e6-334.

2. A low-pressure vapour discharge lamp according to claim 1 in whichthe reservoir is near one of the electrodes of the lamp.
 3. Alow-pressure vapour discharge lamp according to claim 1 in which thereservoir contains a vapourisable substance selected from the groupconsisting of lithium, sodium, potassium, cadmium and mixtures thereof.4. A discharge tube for a low pressure vapour discharge lamp comprising:a light-transmitting envelope; an exhaust tube extending through thewall of said envelope and being sealed to said envelope wall; saidexhaust tube having an end within said envelope that is closed andadapted to constitute a reservoir for vapourisable substance, and saidexhaust tube further having a wall within said envelope formed with anaperture permitting communication between said reservoir and theinterior of said envelope.